Electromagnetic flow control valve for a liquid metal

ABSTRACT

An electromagnetic flow control valve for an electrically conductive liquid having a plurality of coils and yokes surrounding a non-metallic, such as alumina, tube through which an electrically conducting liquid, such as liquid metal, flows. Direct current applied to the coils causes retarding forces to be imposed on the flowing liquid. The electromagnetic flow control valve can be used in conjunction with a continuous caster where the electromagnetic flow control valve is in fluid communication with a tundish.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/037,671, filed on Feb. 11, 1997.

BACKGROUND OF THE INVENTION

1) Field of the Invention

This invention relates to the electromagnetic control of the flow of anelectrically conducting liquid.

2) Description of the Prior Art

Molten metal moves often from one vessel to another during industrialprocesses. Whether it is from a melting or holding furnace to multiplemolds in a batch casting process or from a ladle to a tundish to a moldin a continuous caster, in both the ferrous and non-ferrous industries,the control over the flow of the metal is important or key to theprocess.

The growth of continuous casting in the United States, the emphasis on"clean steel", the rise of ladle and tundish metallurgy, the trend tohigher production machines and the need for precise control ininnovative casting processes have all increased the importance of flowcontrol in molten steel pouring in particular. According to themagazine, 33 Metal Producing, 80% of the steel melted in Americanfurnaces (87 million tons in 1991) passes through a slide gate or valve.An engineering manager of Sumitomo Metal America, Inc. was quoted in 33Metal Producing as stating that if a magnetic field controlled the rateof flow, one could eliminate conventional metering or throttling systemsand reduce costs. Inland Steel has recently cited the possibility of theuse of electromagnetic force to reduce the alumina clogging problem inslide gates. In the steel ingot, casting and non-ferrous metal industry,a similar need is felt. The combined aluminum and copper production inthe United States in 1991 was approximately six million tons.

Today, the state-of-the-art of molten metal flow control in industrialprocesses continues to be by mechanical devices. Three major types ofconventional flow control devices are used at the discharge of afurnace, holding vessel or tundish: a metering nozzle, a stopper rod ora slide gate. A metering nozzle is a specially contoured hole through aceramic block. For a gravity driven flow, the flow rate is simplyproportional to the square root of the head of the molten metal abovethe nozzle and to the square of the nozzle throat diameter. The stopperrod is basically a blunt ended rod suspended above a nozzle andconfigured with a manual or automatic mechanical means for raising andlowering. The flow rate can be varied from fully open to fully closedusing a stopper rod. The slide gate is primarily a hydraulicallyoperated mechanism that basically consists of several stacked ceramicplates, each with a central hole therethrough. The holes may be alignedto allow the flow or misaligned to stop the flow. Both linear and rotaryversions are available. Slide gates are predominately used on furnacesand ladles because of their ability to hold high heads for long periodsof time.

A particularly critical flow control location of great practicalsignificance is from the tundish to the mold in a continuous castingmachine for making steel. As schematically shown in FIG. 1A, a tundish 4is an intermediate, shallow vessel that provides several functions in acontinuous casting machine. Receiving molten metal from a transfer ladlefrom the furnace, the tundish 4 distributes the molten metal throughmultiple bottom openings to individual molds. Multiple ladles may besequenced using the capacity of the tundish as a reservoir. Also, thetundish provides a residence time to allow metal inclusions to floatout. According to a 1986 survey, the metering or free flow nozzle isused on approximately half of the total tundishes in the overall UnitedStates steel industry; while stopper rods and slide gates are each usedon about one-quarter of the steel tundishes, respectively.

One of the prime functions of a tundish is to provide a controlled,uniform flow. A rough stream has a higher surface to volume ratio and,hence, a higher propensity to reoxidize by direct contact with the air.Further, a rough stream will entrain more air and carry it into the moldresulting in disadvantageous turbulence, foaming and sloshing. With aturbulent pool, new steel is continuously brought to the surface forfurther contact with air. Very little time is left for proper separationof impurities. Also, oxides tend to be thrown to the outside of the moldwhere they can be trapped in the surface of the strand. Excessiveturbulence in the molten crater of the strand can also be a potentialcause of a breakout through the shell.

Both stopper rods and slide gates tend to produce rough streams. Also,slide gates and stopper rods are both subject to clogging when castingaluminum killed steels. Toward the end of a sequence cast, the accuracyof flow control gets worse especially with a stopper rod as the flowarea between the rod and the nozzle block becomes fouled. Stream flaringoccurs in a slide gate 95% of the time during a heat sequence when theslide gate needs to be in its semi-open position. The stream exiting thetop portion of the slide gate at an angle translates into a circularmotion through and exiting the slide gate.

Metering nozzles also suffer from operational problems. The only way tocontrol flow with a metering nozzle is to control the tundish levelheight, but this is slow and insensitive being a square root function ofthe head. Other considerations, such as inclusion float time orvortexing, tend to make changes in tundish level undesirable from aquality standpoint. Generally, for a billet caster, nozzle life limitsthe sequence length. The nozzle erodes to the point that the flow rateincreases over the allowable limit for the machine. Also, clogging tendsto limit cast sequencing and, most importantly, the types of steel thatmay be cast. The only way to stop flow through a metering nozzle is tomanually insert a chill plug to freeze the flow. Typically, in the steelindustry, this plug must be burned out with an oxygen lance to restartthe flow, often damaging the nozzle.

A continuous casting operator is market driven to meet one or more ofthe following needs: (1) to meet the quality specifications of thegrades being cast; (2) to diversify by moving into casting improvedgrades of steel; (3) to reduce the current cost of casting a given gradeof steel; (4) to increase the yield of prime billets, i.e., to reducewaste; and/or (5) to upgrade machinery as it ages to continue to competein the market.

An electromagnetic flow control device in lieu of the conventional flowcontrol devices has direct bearing on all of these market drivers. Aspart of an overall caster control system, it does so in a number ofimportant ways to improve process control, improve quality, increaseproductivity and reduce cost.

When compared to metering nozzles, an electromagnetic flow controldevice (1) offers the operator of a billet caster the opportunity to nowcontrol the flow through the caster rather than react to it; (2)provides independent control over the casting rate to meet tightspecifications on the heat removal rates in all commercial grades; (3)provides a greater degree of control over that of changing the tundishlevel height which is slow and insensitive; (4) offers independent flowcontrol on each nozzle to compensate for uneven nozzle wear or cloggingamong the multiple strands in a caster fed from the same tundish; and(5) gives the operator the capability to adjust flow independent ofstrand motion changes to maintain a constant mold level height which isso important to good quality.

Since 95% of the flow control in the mini mill market is by meteringnozzle, these advantages are particularly important. The mini millsector is no longer just the low cost producer of rebar. Higher qualitybillets for structural shapes and special bar quality (SBQ) are beingproduced regularly. The ability to counter the traditional nozzleblockage problem of aluminum killed steels via the controlled flow andheat addition capability of an electromagnetic nozzle opens up marketscurrently not available to the mini mills. In the mini mill industry, anumber of billet casters were put into service originally in the 1960sand 1970s. These machines are in need of upgrades to the currentstate-of-the-art to compete today.

When compared to stopper rods or slide gates, electromagnetic flowcontrol (1) regulates flow without introducing stream roughness and thesubsequent mold turbulence, reoxidation and impurity entrapment; (2)eliminates sites where stream velocity changes abruptly which thencauses inclusions to accumulate; (3) eliminates the mechanisms needed tomove the rod or plates which are subject to wear and failure; and (4)allows a higher number of sequential casts through increased nozzlereliability and performance which results directly in higherproductivity and cost savings.

An electromagnetic flow control device can perform a number ofbeneficial functions that current metering nozzles, stopper rods orslide gates cannot by: (1) electromagnetically improving the steadinessof the pouring stream eliminating turbulence from the ladle stream andthe tundish resulting in improved quality; (2) providing additional heatdirectly at the nozzle to reduce the tendency for inclusion depositionand the possibility of freezing; (3) applying heat at the nozzleelectromagnetically to remelt a strand if it has been deliberatelyfrozen off avoiding the damage typically done by an oxygen lance; (4)giving a greater capacity to deal properly with hot or cold heats; and(5) permitting larger tundishes having a greater depth to minimizevortexing and to maximize inclusion float through offsetting the extrahead caused by the larger tundishes.

Electromagnetic flow control is also believed to improve performanceover the current mechanical flow control in the following ways: (1) itpermits a computer to have better and more responsive control over themetal flow rate through the caster to better match the furnace; (2) itachieves more uniformity from cast to cast by reducing the reliance onindividual operator's skill and potentially the number of operatorsneeded; and (3) it reduces costly events, such as breakouts, nozzlelancing and caster turnarounds, with the subsequent upturn in strandyield.

An early reference in the metals industry to the use of electromagneticflow control was in 1960. It involved the use of a 300 kilowatts, 3,000hertz induction tundish heating system with two coils, one for main bodyand one for the spouts. While the body coil primarily provided constanttemperature control of the metal, the spout coil provided an additionalstabilizing effect on the pouring stream preventing splashing and thus,helping to maintain a "quiet" level in the top of the mold. Morerecently, Garnier at Grenoble, "Liquid Metal Flows andMagnetohydrodynamics", Progress in Astronautics and Aeronautics, Volume84 (1981), experimented using mercury with high frequency alternatingcurrent (ac) electromagnetic devices to achieve convergent or divergentflows in a vertical molten metal column. Takeuchi et. al., at the 1992TMS Symposium on Magnetohydrodynamics in Process Metallurgy summarizeddescriptions of linear and rotary ac motors to control the pouring ratefrom a vessel. Kirillov and Vitkovsky, at the Nagoya InternationalSymposium of Electromagnetic Processing of Materials (1994), discussedRussian electromagnetic brakes for liquid metal flow regulation.

U.S. Pat. No. 2,707,720 discloses a container with an opening in thebottom near the wall and surrounded by an electric coil. An alternatingcurrent applied to the coil forces the molten metal to move away fromthe opening by an induced magnetic pressure. U.S. Pat. Nos. 3,463,365and 3,701,357 describe devices where an external current is passedthrough a liquid metal, which then interacts with an externally imposedmagnetic field to generate a force component retarding the flow ofliquid metal. U.S. Pat. No. 3,695,334 describes the use of a rotatingelectromagnetic field to generate rotational motion and a radialpressure gradient in a container with a liquid metal inlet at the outerperiphery and exit at the central axis. U.S. Pat. Nos. 4,082,207 and4,324,266 disclose an alternating current winding and an electricallyconductive screen to constrict the jet of molten metal at the outlet ofa nozzle. U.S. Pat. Nos. 4,805,669 and 4,947,895 discloseelectromagnetic valves with specially shaped internal dischargepassageways surrounded by induction coils supplied with a highfrequency, alternating current. U.S. Pat. No. 4,842,170 teaches a devicewith an alternating current electromagnetic coil surrounding a nozzleorifice with a central portion designed to allow eddy currents to flowin certain regions and not in others resulting in an axially directedforce to impede the flow. Finally, U.S. Pat. No. 5,137,045 discloses analternating current electric coil surrounding a descending stream tooptimize magnetic pressure versus power loss.

All of the devices cited above operate by combining a magnetic fieldvector B and an electric current density vector J to generate a bodyforce vector F via the vector Lorentz Law, F=J×B. The fields andcurrents may be alternating (ac) in time and/or in space or steady (dc).The current may be internally generated by induction or by an externallyapplied electric potential. U.S. Pat. Nos. 2,707,720; 4,082,207;4,324,266; 4,805,669; 4,947,895; 4,842,170; and 5,137,045 utilize singlecoils supplied with high frequency alternating current to create a timevarying, spatially fixed magnetic field. The time variation of themagnetic field results in an electric field according to Maxwell'sEquations. In an electrically conducting fluid, the electric fieldcauses eddy currents to flow in the fluid. As stated above, theinteractions of the eddy currents and the imposed magnetic field resultin electromagnetic body forces exerted on the fluid. The steadycomponent of the body force effectively confines or retards the flow asdesired. The time alternating component is not generally useful since itchanges too fast for the fluid to follow. The Takeuchi et al. articleand U.S. Pat. No. 3,695,334 describe multi-coil, multi-phase ac deviceswhere the magnetic field moves in a rotary or linear fashion. Here, thebody forces from the eddy currents try to make the fluid catch up withthe field, analogous to the slip of the rotor in a conventionalinduction motor. U.S. Pat. Nos. 3,463,365 and 3,701,357 use thevariation of applying an external electric potential to generate thecurrent.

Most of the systems described in the technical and patent literaturehave inherent practical limitations. Given the actual metal types, sizesand flow rates of industrial metal pouring situations, devices which usean individual coil carrying a single phase current, such as described byU.S. Pat. Nos. 4,805,669; 4,947,895; 4,082,207; or 4,324,266, need tooperate at high frequencies, typically from several thousand to tens ofthousands of cycles per second. This is basically to match the skindepth of field penetration where the eddy currents flow relative to thesize of the flow stream. Special power supplies are needed to generatethe high frequency current from the standard power line supply.Inductive and capacitive matching is needed between the supply, thecoil, and the molten metal load. The real and reactive power that needsto be supplied to the system to overcome the losses of the applied andinduced eddy currents is in general very high, often hundreds ofkilowatts. Buswork connecting the coil to the supply must be low inresistance and inductance, meaning large parallel copper buswork orcoaxial copper cables. Generally, because of the high power, the coiland the power cabling must be water cooled. Cooling the coil is alreadydifficult due to the necessity to have it in very close proximity to theelevated temperature of the molten metal. Some devices, such asdescribed by U.S. Pat. No. 4,842,170, also need to insert a speciallyshaped, non-conducting plug into the flow passage to direct the metalflow or eddy currents in a special way. Such plugs are subject toerosion, clogging or thermal failure, especially when dealing withmolten steel. The transfer of electrical current into a device, such asdescribed by U.S. Pat. No. 3,463,365 for a high temperature moltenmetal, such as steel, is hampered by the lack of suitable electrodematerials. The currents must be high to effect the level of retardingpressure required. The point in a metal pouring process where flowcontrol is desired is unlike that of an electric arc furnace where highlosses, heating and arcing can be tolerated. The rotating field devicedescribed in U.S. Pat. No. 3,695,334 requires high fluid peripheralvelocities to offset practical heads and is not suitable where the flowis desired to be as quiescent as possible. The flow passages in metalpouring are, in general, too small for linear ac pumps to be practical.

All of these factors tend to make these kinds of systems complicated andexpensive. These factors have severely limited their practicalapplication for flow electromagnetic control.

Baker, "Design of an Eddy-Current Brake for a Sodium-Cooled NuclearPower Reactor", AIEE Winter Meeting (1960), New York, N.Y., describesthe use of a rectangular directing current (dc) flow brake to retard theflow in a liquid metal reactor after a shutdown or scram. Baker's deviceincludes a flattened section of stainless steel pipe suspended betweenthe poles of a conventional C-shaped, iron electromagnet. The workingfluid is sodium. Both the rectangular shape of the flow passage and thestainless steel material for the tube are not readily practical for mostmetallurgical metal pouring situations. Shercliff, The Theory ofElectromagnetic Flow Measurement, Cambridge University Press (1962),describes a variation of this dc device. It includes a singleaxisymmetric coil surrounding a round pouring tube. An iron flux returndonut surrounds the coil reducing the reluctance of the magneticcircuit. In practical application for controlling the flow of a steelstream exiting from a tundish, Shercliff's device does not allowsufficient space for the electric coil. As the resistance losses dependon the available cross-sectional area of the coil, as disclosed byShercliff, the axisymmetric device also requires an impractical highpower to operate. The coil is also located directly next to the pouringtube which will be at an elevated temperature. Cooling of the coilbecomes difficult due to the heat transferred from the tube andinternally generated by the resistive losses.

As described below, the present invention overcomes the disadvantages ofthe ac induced and externally applied current flow control devices citedearlier. The present invention also improves on and eliminates thedeficiencies of the dc flow brakes known to date. Further objects andadvantages of the invention will become apparent from a consideration ofthe drawings and ensuing description.

Therefore, it is an object of the present invention to provide anelectromagnetic flow control device that does not require high frequencyalternating current for operation. It is a further object of the presentinvention to provide an electromagnetic flow control device that doesnot require any kind of special internal passage member to direct theflow. It is a further object of the present invention to provide anelectromagnetic flow control device that is compact in size, readilymanufacturable and is low in cost. It is another object of the presentinvention to provide an electromagnetic flow control device that doesnot require high power for operation.

SUMMARY OF THE INVENTION

My invention is an electromagnetic flow control valve that includes atube and a plurality of electric coils. The tube defines a centralpassageway adapted to permit an electrically conductive liquid to passtherethrough. The plurality of electric coils is positioned about acircumference of the tube. The coils are formed of electricallyconductive wire whereby passing electric current through the wires ofthe coils causes a magnetic field to be formed in the passageway whichretards a flow of electrically conductive liquid through the centralpassageway.

The tube is transparent to a magnetic field and can be made of a ceramicmaterial, such as alumina.

Each of the coils defines a coil passageway and each of the coilsreceives a core in the coil passageway. The core includes a highpermeability material, such as iron.

Preferably, end plates are provided and each of the coils has a firstend and a second end where the end plates sandwich the coils between thefirst ends and the second ends. Each of the end plates is made of a highpermeability material, such as iron. The end plates can include aplurality of projections, such as lobes. Each of the coils receives acore in a coil passageway defined in respective ones of the coils.Respective ones of the cores coact with a respective pair of theprojections.

Two longitudinally spaced apart annular shaped pole rings attach torespective ones of the end plates. The tube passes through the polerings.

My invention is also a continuous caster for an electrically conductingliquid that includes a ladle, a tundish in fluid communication with theladle and the above-described electromagnetic flow control valve wherethe tube is in fluid communication with the tundish.

My invention is also a method for controlling the flow of anelectrically conducting liquid that includes the steps of: passing anelectrically conducting liquid through a tube that is transparent to amagnetic field; directing a plurality of circumferentially positionedmagnetic fields toward the tube; and controlling the flow of theelectrically conducting liquid by the strength of the magnetic fields.The method can further include that the circumferentially spacedmagnetic fields are provided by a plurality of circumferentially spacedelectric coils.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a continuous casting machine for steelhaving an electromagnetic flow control valve made in accordance with thepresent invention;

FIG. 1B is a perspective view, partially in section, of a continuouscaster similar to the caster shown in FIG. 1A having only oneelectromagnetic flow control valve made in accordance with the presentinvention;

FIG. 2 is a graph of percent flow versus magnetic field strength for asteel continuous caster having an electromagnetic flow control valve;

FIG. 3 is a top plan view, partially in section, of the electromagneticflow control valve shown in FIG. 1A;

FIG. 4 is a section taken along lines IV--IV of FIG. 3;

FIG. 5 is a section taken along lines V--V of FIG. 3 showing a patternof magnetic flux lines, eddy currents, velocities and forces on metalflowing through a central tube of the electromagnetic flow controlvalve;

FIG. 6 is a section taken along lines VI--VI of FIG. 1A;

FIG. 7 is a top perspective view of a portion of a second embodiment ofan electromagnetic flow control valve;

FIG. 8 is an elevational view in section of the second embodiment of theelectromagnetic flow control valve shown in FIG. 7 contained within acasing; and

FIG. 9 is an elevational view in section of a third embodiment of anelectromagnetic flow control valve made in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic equations for the electromagneto dynamics of fluids may bedescribed using the standard magnetohydrodynamics (MHD) approximations.Maxwell's Equations become: ##EQU1##

Ohm's Law is:

    J=σ(E+V×B)

The magnetic constitutive equation is:

    B=μ.sub.0 (H+M)

The fluid equations for continuity and motion are standard except forthe addition of the J×B body force term: ##EQU2##

The prior art devices described earlier primarily generate the currentdensity J through the electric field term E in Ohm's Law. As statedbefore, E comes from the variation of the magnetic field B with time oris applied externally. In general, in these devices, the back emf, i.e.,the V×B term in Ohm's Law, is of little or secondary influence. However,there is a class of devices where it becomes the primary term togenerate the current density J and, hence, the body force F. Thesedevices are known as eddy current brakes.

Since the flow and field are steady state, the derivative terms withrespect to time vanish in the above set of equations. As the problem isalso axisymmetric, any peripheral variations are zero.

The body force F, magnetic field B and velocity V can be simply relatedin the axisymmetric dc device as follows: ##EQU3## where r and z arecomponent designations and σ is the fluid electrical conductivity. Sincethe primary velocity in the device is in the axial z direction, theretarding force, -Fz, is to first order proportional to the fluidconductivity, the axial velocity, and the square of the radial field. Ascan be seen from the above matrix equation, there is some cross couplingof the radial and axial fields and flows. I have conducted detailednumerical modeling of the problem to date which has shown that thecoupling effects are of a second order.

Shercliff gives an order of magnitude estimate for the flow drag in aaxisymmetric device as follows:

    ΔP=σB.sup.2 Va

ΔP=Pressure Drag

σ=Conductivity

B=Radial Magnetic Field

V=Velocity

a=Pipe Radius

This formula assumes that the axial extent of the field is approximatelythe same as the pipe diameter and that the average value of the radialmagnetic field squared is one-half of the maximum. The Table below showsthe estimated pressure drag for steel, aluminum and copper in somereference casting situations where some of the head in the tundish isoffset by the drag through the flow control device.

                  TABLE                                                           ______________________________________                                                                  Magnetic                                                                                    Pressure                                              Resistivity                                                                        Velocity                                                                            Field    Diameter                                                                           Drag                                 Metal       (μΩ-cm)                                                                    (ft/sec)                                                                               (T)        (in)                                                                                 (psi)                            ______________________________________                                        Steel  137       6.6      0.8    3.1    4.8                                   Aluminum                                                                                 20.5                 0.4                                                                                 3.1                                                                                   2.3                             Copper       21                 1.0                                                                                 1.6                                                                                   7.0                             ______________________________________                                    

The control of the flow in a steel continuous caster is one of the mostdifficult applications for several reasons. The liquidus temperature ofmolten steel is significantly higher than non-ferrous metals. Theelectrical resistivity of molten steel is about seven times that ofmolten aluminum or copper. Molten iron is also approximately three timesas heavy as molten aluminum. Heads to be offset in steel casting canalso be higher than for other liquid metals. All of the above-describedmetals are non-magnetic in the molten state. All of these factorsrequire a significantly higher retarding head to be produced for thesteel application. Hence, a practical device for steel would then haveready application for many non-ferrous metals.

FIGS. 1A and 1B show the flow of steel in a modern continuous casterincorporating the present invention. Liquid molten steel 12 from ladle2, which is in fluid communication with a tundish 4, pours first intothe tundish 4. From the tundish 4, the liquid molten steel 12, in turn,pours into molds 6. For a slab caster, there may only be one nozzle exitper tundish; while for a bloom or typical billet caster, one tundish canfeed several molds simultaneously as shown in FIG. 1A. Electromagneticflow control valves 8 fit to the bottom of the tundish 4 and surroundeach of the discharging liquid molten steel streams 12.

FIG. 2 shows how the magnetic field would be varied according toShercliff's formula to control the casting rate from 100% to 10% in anexample dual strand slab caster at, for example, the USS Edgar ThomsonPlant, Braddock, Pa. This caster has an annual capacity of 2.6 milliontons of steel slabs from 8 to 10 inches thick by 28 to 65 inches wide.The normal operating head in the tundish is 48 inches. The flow controlis quite good, down to about 10% maximum flow rate. Below that, thefields required become large relative to those generated convenientlywith standard iron cores. Although the flow cannot be made to go to zeroelectromagnetically, this in practice is not a problem. A simple,mechanical backup, such as a guillotine gate, for complete shutoff wouldbe provided for safety.

FIGS. 3, 4 and 6 show the electromagnetic flow control valve 8 made inaccordance with the present invention. Liquid molten steel 12 or otherelectrically conductive liquid flows or passes for reference downwardunder gravity through a central tube 14 extending along a longitudinalaxis Z. The central tube 14 is in fluid communication with the tundish4. The central tube 14 includes a central passageway P and is straightand of a constant thickness as conventionally employed in casting. It ispreferably made of a standard ceramic material, such as alumina. Assuch, it can withstand the high temperature of the liquid molten steel12; yet be transparent to the magnetic field. Importantly, noobstruction needs to be placed on the inside of the central tube 14.Radially surrounding the central tubes 14 are a multiplicity orplurality of assemblies 15 each having an electric coil 16, a coil core10 and end plates or yokes 18. The electric coils 16 are torrid shapedand made of wound electrically conductive wire or copper wire in amanner well known in the art and are disposed a distance X from the hotcentral tube 14 reducing radiant and convective heat transfer. Aplurality of electric coils 16 is positioned circumferentially C fromthe central tube 14 and a radial distance R from the tube. Anintermediate metal heat shield 20 made of reflective stainless steel ora highly thermally resistive material, such as woven ceramic fibers, canbe placed around the central tube 14 to help reduce the heat load. Eachof the electric coils 16 defines a coil passageway CP that receives thecoil core 10. Each of the electric coils 16 includes a first end a and asecond end b. The end plates 18 sandwich the electric coils 16 betweenthe first end a and second end b of the electric coils 16. The endplates 18 and coil cores 10 are preferably composed of a highpermeability material, such as iron. Since the magnetic field is steady,the magnetic structure need not be laminated for electrical reasons.Each of the end plates 18 is connected to the coil cores 10. Each endplate 18 connects to or attaches to one of two interior pole rings 22.The pole rings 22 are specially shaped, in this case, annular shaped orring shaped, to direct the flux into the central tube 14 both axiallyand radially. The pole rings 22 may be made of a higher permeabilityalloy, such as iron cobalt, since the magnetic flux density is highestat these places. The central tube 14 passes through the pole rings 22through holes H defined by the pole rings 22. A shell or casing 24surrounds the assemblies 15 and provides mechanical support andintegrity for the entire device. A suitable attachment arrangement, suchas bolts, pins, plugs or any other type of attaching arrangement, isprovided to the exterior of the shell 24 to the bottom of the tundish 4.

FIG. 5 shows patterns of magnetic field lines B designated as referencenumeral 30, eddy currents J designated as reference numeral 32, axialbody forces F designated as reference numeral 34 and velocity vectors Vdesignated as reference numeral 36 on a radial plane passing through thedevice. The axial body forces 34 are generated primarily in the endregions of the electromagnetic flow control valve 8 to retard the flowin the end regions. The end shape of the pole pieces is arranged tomaximize the radial direction of the magnetic field lines 30. The eddycurrents 32 flow in a peripheral direction making closed loops. Theaxial body forces 34 being the vector cross product of the currentdensity and magnetic field are mutually perpendicular to both the fluxline and eddy current 32 at each point. Although the directions of themagnetic flux and eddy currents are opposite in each end region, thegenerated axial body forces 34 always oppose the flow. From the matrixequation, the sign of Br, the radial flux component does not mattersince the force depends on Br². Because of the minus sign, the forcealways opposes the flow.

Referring back to FIGS. 4 and 6, four assemblies 15 are shown. Each ofthe end plates 18 includes four projections, such as four lobes 38,where each electric coil 16 is sandwiched by a pair of lobes 38 of endplates 18. Each of the coil cores 10 coact with a respective pair of thelobes 38, such as by contacting the lobes 38 or being in close proximityto the lobes 38 so that the lobes 38 electromagnetically coact with thecoil cores 10. Each of the lobes 38 is secured to each other for arespective end plate 18. Each assembly 15 includes a spaced apartelectric coil 16 and two spaced apart lobes 38. The number of lobes 38and electric coils 16 can vary from case to case. Depending upon theparticular design, the electric coils 16 may be wired electrically inseries, in parallel, or some combination thereof and connected to apower supply. It is preferred to use a standard 125 volt dc or 250 voltdc power supply that is readily available industrially at many kilowattratings. The electric coils 16 are arranged so that passing electriccurrent through the wire of the electric coils 16 causes a magneticfield to be formed in the central passageway P to retard a flow of theliquid molten steel 12 or other electrically conductive liquid throughthe central passageway P. The electric coils 16 may be constructed ofsolid conductor and externally cooled. Preferably, the coils areaxisymmetric about axes Z', Z", Z'" and Z"" which are parallel to the Zaxis and spaced an equidistance. Alternately, they may be constructedwith hollow conductor and internally cooled by gas or liquid. It islikely that liquid cooling with, for example, a water-glycol mixture, ispreferred for the steel use. As mentioned above, pole rings 22 serve todirect the magnetic flux into the liquid molten steel 12. A feature ofthe axisymmetric configuration of the present invention is the increasein the flux density in the region of the central tube 14 and liquidmolten steel 12. All of the magnetic flux from each lobe 38 is directedto a respective quadrant of the liquid molten steel 12 and the centraltube 14. Since the lobes 38 are evenly spaced about the central tube 14and, in this case, four lobes 38 are provided for each end plate 18, themagnetic flux of each lobe 38 is directed to one-quarter or one quadrantof the liquid molten steel 12 and the central tube 14. All of themagnetic flux in each of the end plates 18 flows through a singlequadrant of the liquid molten steel 12 and the central tube 14. Sincethe total amount of flux in each magnetic path is constant (equal to theampere turns in each coil divided by the reluctance of the circuit) asthe area decreases with r, the flux density B increases.

In operation, the flow of liquid molten steel 12 through the centraltube 14 is controlled by the magnetic field as shown in FIG. 2. This iscontrolled by the electric power passing through the coils. Morespecifically, an electrically conducting liquid's flow can be controlledby passing an electrically conducting liquid through a tube that istransparent to a magnetic field; directing a plurality ofcircumferentially positioned magnetic fields toward the tube; andcontrolling the flow of the electrically conducting liquid by thestrength of the magnetic fields. The circumferentially spaced magneticfields can be provided by a plurality of circumferentially spacedelectric coils positioned about the tube. The magnetic field iscontrolled by the electric power passing through the coils, increasingthe power to increase the magnetic field and decreasing the power todecrease the magnetic field.

FIGS. 7-9 show alternate embodiments of electromagnetic flow controlvalves 8' and 8" with the use of more than one assembly 15 in thelongitudinal direction. Like reference numerals are used for likeelements. Greater amounts of flow retardation may be accomplished byvertically stacking end plates and coil assemblies 15. For redundancyreasons, each layer 100 may be separately powered. The coil in eachlayer may have the current flow in the same or in the opposite sense tothe coils in the adjacent layer. These embodiments operate in the samemanner as the previous electromagnetic flow control valve 8. Since theonly difference between the embodiments shown in FIGS. 7-9 is thisstacking feature, no further discussions are necessary.

As is now evident, the electromagnetic flow control valves 8, 8' and 8"do not require high frequency alternating current for operation; do notrequire any kind of special internal passages to direct flow; arecompact in size, readily manufacturable and are low in cost; and do notrequire high power for operation.

Having described the presently preferred embodiments of the invention,it is to be understood that it may otherwise be embodied within thescope of the appended claims.

I claim:
 1. An electromagnetic flow control valve for molten metal,wherein the metal is non-magnetic in the molten state, comprising:atube, said tube defining an unobstructed central passageway adapted topermit a molten metal to pass therethrough; and a plurality of electriccoils positioned circumferentially about said tube, said coils formed ofelectrically conductive wire whereby passing electric current throughsaid wires of said coils causes a magnetic field to be formed in saidcentral passageway which retards a flow of molten metal through thecentral passageway.
 2. An electromagnetic flow control valve as claimedin claim 1, wherein said tube is transparent to a magnetic field.
 3. Anelectromagnetic flow control valve as claimed in claim 2, wherein saidtube comprises ceramic material.
 4. An electromagnetic flow controlvalve as claimed in claim 3, wherein said ceramic material comprisesalumina.
 5. An electromagnetic flow control valve as claimed in claim 1,wherein each of said coils defines a coil passageway and each of saidcoils receives a core in said coil passageway.
 6. An electromagneticflow control valve as claimed in claim 5, wherein said core comprises ahigh permeability material.
 7. An electromagnetic flow control valve asclaimed in claim 6, wherein said high permeability material comprisesiron.
 8. An electromagnetic flow control valve as claimed in claim 1,further comprising end plates, each of said coils having a first end anda second end, said end plates sandwich said coils between said firstends and said second ends.
 9. An electromagnetic flow control valve asclaimed in claim 8, wherein each of said end plates comprises a highpermeability material.
 10. An electromagnetic flow control valve asclaimed in claim 9, wherein said high permeability material comprisesiron.
 11. An electromagnetic flow control valve as claimed in claim 8,wherein said end plates comprise a plurality of projections secured toeach other.
 12. An electromagnetic flow control valve as claimed inclaim 11, wherein said end plates projections are lobes.
 13. Anelectromagnetic flow control valve as claimed in claim 11, wherein eachof said coils receives a core in a coil passageway defined in respectiveones of said coils and respective ones of said cores coact with arespective pair of said projections.
 14. An electromagnetic flow controlvalve as claimed in claim 8, further comprising two longitudinallyspaced apart annular-shaped pole rings, said pole rings attached torespective ones of said end plates and said tube passing through saidpole rings.
 15. An electromagnetic flow control valve as claimed inclaim 14, wherein each of said end plates comprises a plurality ofprojections, wherein pairs of projections receive respective ones ofsaid electric coils.
 16. An electromagnetic flow control valve asclaimed in claim 15, wherein each of said coils receives a core in acoil passageway defined in respective ones of said coils and respectiveones of said cores coact with said respective pairs of said projections.17. A continuous caster for for molten metal, wherein the metal isnon-magnetic in the molten state, comprising:a ladle; a tundish in fluidcommunication with said ladle; and an electromagnetic flow controlvalve, comprising:a tube, said tube defining an unobstructed centralpassageway adapted to permit a molten metal to pass therethrough; and aplurality of electric coils positioned about a circumference of saidtube, said coils formed of electrically conductive wire whereby passingelectric current through said wires of said coils causes a magneticfield to be formed in said passageway which retards a flow of moltenmetal through the central passageway, wherein said tube is in fluidcommunication with said tundish.
 18. A continuous caster for anelectrically conducting liquid as claimed in claim 17, furthercomprising end plates, wherein each of said coils having a first end anda second end, said end plates sandwich said coils between said firstends and said second ends.
 19. A method for controlling the flow ofmolten metal, wherein the metal is non-magnetic in the molten state,comprising the steps of:passing molten metal through a tube having anunobstructed passageway that is transparent to a magnetic field;directing a plurality of circumferentially positioned magnetic fieldstoward the tube; and controlling the flow of the molten metal by thestrength of the magnetic fields.
 20. A method for controlling the flowof an electrically conducting liquid as claimed in claim 19, wherein thecircumferentially spaced magnetic fields are provided by a plurality ofcircumferentially spaced electric coils.